Silicon nitride powders all have a certain oxygen content. Depending on the field of application, the oxygen content is about 0.8 to 2.5% by weight. Although the oxygen is strictly speaking an impurity of the powder, it is regarded as necessary to a certain extend for improving the sintering activity of the silicon nitride powder (G. Ziegler, J. Heinrich, G. Wotting, J. Mater. Sci. 22 (1987), 3041-86).
Sintering additives are conventionally added to the silicon nitride powder and together with the oxygen in the powder they form liquid phases at the sintering temperature and are essential for bringing about a compacting of the material. The oxygen content in the silicon nitride powder should, however, not be too high since a high oxygen content lowers the glass temperature of the secondary phases formed in the sintering process and therefore impairs the mechanical properties at high temperatures of the sintered product obtained. An oxygen content of about 1.5% by weight is generally regarded as optimal for gas pressure sintering but for hot isostatic pressing the oxygen content may be lower. No limiting values are known for the oxygen required for hot isostatic pressing.
Determination of the oxygen distribution in commercial Si.sub.3 N.sub.4 powders by ESCA (Electron Spectroscopy for Chemical Analysis) is described in J. Mater. Sci. 22 (1987), 3717-3720. It is stated there that the oxygen content should be reduced in the interior of the particles. For a required total oxygen content of 1.5% by weight, this means an enrichment of oxygen on the surface of the particles. According to the said literature reference, the distribution of oxygen between the interior of the particles and the surface of the particles depends to a large extent on the method of preparation employed. Si.sub.3 N.sub.4 powders prepared by the nitridation of silicon or by reductive nitridation of SiO.sub.2 show only a slight enrichment of oxygen on the surface of the powder. The Proportion of surface oxygen is from 17% to 58% of the total oxygen content. In powder prepared by the gas phase reaction of SiCl.sub.4 and NH.sub.3 at room temperature (Toyo Soda TS 7), the surface oxygen content is 60%. In Si.sub.3 N.sub. 4 powder obtained by the liquid phase reaction between SiCl.sub.4 and NH.sub.3 followed by thermal decomposition of the diimide (Ube E 10), the enrichment of oxygen on the surface is even higher, amounting to 83%. This powder, however, has a total fluorine content, determined under wet chemical conditions, of over 35 ppm. In ESCA investigations, the fluorine content on the surface of the powder is about 0.3 Atom-%. Fluorine, however, reduces the high temperature strength of parts obtained by sintering the powder since fluorine influences the glass temperature of the secondary phases in the same way as oxygen (L. A. G. Hermansson, M. Burstroom, T. Johansson, M. E. Hatcher, J. Amer. Ceram. Soc. 71 (4) (1988), C183-184).
Apart from the general methods for the preparation of powders, there are several methods for adjusting the oxygen content on the surface of the powders. Firstly, the powder may be partly oxidized in air by annealing at temperatures above 500.degree. C. (see Greskovich, J. A. Palm, Am. Ceram. Soc. Bull. 59(11) (1980), 1133). The oxidation begins on the surface of the powder, thereby enabling the surface oxygen content to be increased. This, however, leads to an increase in the oxygen content to over 1.8% by weight and hence, as described above, to an impairment of the high temperature properties. The method of increasing the surface oxygen content by hydrolysis has the same disadvantage. When commercial Si.sub.3 N.sub.4 powder having a low surface oxygen content is ground up in water or alcohol, a surface layer rich in oxygen is obtained by hydrolysis but at the same time the total oxygen content is increased to a value above 1.8% by weight.
If one starts with powders which have a high oxygen content on the surface, obtained, for example, by grinding in water, the total oxygen content may be reduced by leaching with HF. By optimization, it is possible to obtain a powder which has a high surface oxygen content and a total oxygen content of less than 1.8% by weight. This method has, however, the disadvantage that traces of fluoride ions are left in the powder and accumulate on the surface of the powder as a result of the procedure employed. These traces cannot be removed by washing.
It is an object of the present invention to provide Si.sub.3 N.sub.4 powders which do not have the above-described disadvantages of the state of the art.